Use Special Invasion Ability To Grind XP Very Fast (Patreon)

One strategy I have been using in Legion for some time to level mobs is to use special quest abilities to level much faster. 

Invasions in particular have high mob densities, and large numbers of players allowing the player to piggyback other player kills and cross-realm quickly when all the mobs in the area have been killed. When you add in a special ability to kill mobs faster this leads to a perfect storm for mob grinding XP.

The best example of this is the Legion Assault World Quest "Borne Of Fel" in Highmountain. In this quest you free a drogbar named "Norgabbon Of Stonedark" who gives you an ability known as "Norgabbon's Tectonic Totem". 

This deals massive damage and will one-shot most of the mobs in this area. Aside from the Invasion quests themselves this is the fastest way I know of to level in Legion currently.

There are many similar Invasion Quests which allow the player to use some OP ability temporarily though I haven't found anything as efficient as this one.